Conventional hot strip mills have been designed for rolling slabs from a primary slab mill which is capable of providing slabs of any desired width for subsequent continuous rolling. Continuous slab casters, however, operate most efficiently at a constant slab width and thickness. Dimensional changes are accomplished by changing molds, which is time consuming, and requires a considerable mold inventory, or by using molds of adjustable dimensions, which molds are costly and give rise to difficulties not encountered by molds of fixed dimensions. It is pointed out in Stone U.S. Pat. No. 3,580,032 that to cast slabs in a width range of 32 inches to 74 inches requires as many as 20 molds.
Efforts have therefore been made to reduce the widths of continuously cast slabs by edge rolling or pressing prior to thickness reduction, both of which techniques are described in the Stone patent above mentioned. Reduction of a few inches can be made by edge rolling without too much shape distortion but heavy edge rolling of hot slabs thickens the slab edges with respect to its center portion, which effect is known as "dog-boning". A schematic cross section of a slab so rolled is shown in FIG. 6 hereof. When that slab is rolled through the following thickness-reducing stand, however, some of the excess metal at the ends of the dog-bone is rolled out transversely, so that the net reduction in product width is appreciably less than that provided by the edging pass. This is shown in FIG. 7 hereof. The remaining excess metal is rolled longitudinally to a somewhat greater length along the edges of the slab than at its center, producing the "fishtail" shown in FIG. 5 hereof. This fishtail has to be cropped and so appreciably reduces the yield from the slab.
Grooved edging rolls produce somewhat better results than cylindrical rolls but require the groove contour to be matched to the slab thickness. In the Stone patent edger rolls with grooves of adjustable width are provided for that purpose. While the width of the groove can be changed in that way, the angle of inclination of the sides of the groove to its base are fixed.
It has also been proposed to upset or forge the trailing end of a slab transversely to create voids which will fill in during subsequent horizontal rolling so as to minimize or eliminate fishtailing. Again the shape of the void varies with the width of the slab and the amount of width reduction required, and a considerable inventory of dies for that purpose must be maintained.